The present invention relates to a power transmission apparatus used in a vehicle etc.
A power transmission apparatus used in a vehicle is disclosed in Japanese Laid-Open Patent No. 50-158765 for example. In such an apparatus, torque is transmitted between a plurality of rotary plates closely spaced and relatively rotated with respect to each other within a sealed chamber by means of the shearing resistance of a viscous fluid contained in the chamber and filling the spaces between the rotary plates. Such a structure is called a viscous coupling combined with a center differential device etc. for a vehicle and is used as a differential restricting device.
In the center differential device, the vehicle can run while absorbing the differential rotation generated between the front and rear wheel shafts when cornering. However, when the vehicle travels over muddy or slippery terrain, and one of the front and rear wheels begins to slip, the transmitted torque is greatly reduced and the slipping wheel rotates at a high speed, and the other wheels are stopped by the great reduction of the transmitted torque. As a result, the vehicle tends to become stuck. The differential function is restricted to improve such a situation.
In the situation mentioned above, the rotary plates connected to the front and back wheel shafts are relatively rotated by the state in which one of the wheels slips and the other wheels are stopped so that the shearing resistance of the viscous fluid is generated, thereby generating the action for restricting the relative rotation of the rotary plates. Thus, the one of the wheels is prevented from the slips and the transmitted torque is increased and the other wheels are rotated so that the vehicle can get underway.
However, in such a power transmission apparatus or the viscous coupling, the temperature within the sealed chamber is increased by the mutual rotation of the resistance plates, thereby increasing the pressure in the sealed chamber. When the temperature in the chamber has reached a predetermined temperature, as shown in the Japanese Laid-Open Patent, the transmitted torque between the resistance plates tends to rapidly increase. This is called a hump phenomenon which is important to determine the performance of the viscous coupling.
It is advantageous to increase the transmitted torque by increasing the pressure in the chamber due to the increase of the temperature wherein when the differential restricting function is greatly needed. However, it is possible that such as phenomenon occurs when it is not needed.
Further, in the conventional apparatus, it is difficult to measure the pressure within the sealed chamber since the sealed chamber is disposed within a rotary portion of the apparatus. In addition, the performance of the viscous coupling is determined when the viscous coupling is assembled, and the occurrence of the hump phenomenon depends on viscous couplings.
Furthermore, in the conventional power transmission apparatus comprising a viscous coupling or a viscous clutch, the viscous fluid such as silicon oil is filled within the apparatus at a predetermined rate after the assembly thereof. However, in such a conventional apparatus, the characteristics of the transmitted torque is determined by the filling rate of the viscous fluid at the initial stage so that it is possible to generate a phenomenon for braking the transmitted torque.